This invention relates generally to thermoplastic films suitable for cook-in packaging, and more particularly to cook-in food films having food contact surface characteristics which promote binding adherence to a contained food product during cook-in. The films of the invention exhibit improved heat seal strength as compared to known films having a Surlyn food contact layer that bindingly adheres during cook-in to a contained meat product.
The food packaging industry needs a packaging film from which bags and casings can be made which are of improved structural soundness such that they may be fully characterized as cook-in. Further, a precooked food product attractively packaged inside the film within which it was precooked is desirable. The term "cook-in" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to cook-in time-temperature conditions while containing a food product. Cook-in packaged foods are essentially foods cooked in the package in which they are distributed to the consumer and which may be consumed with or without warming. Cook-in time-temperature conditions typically refer to a long slow cook, for example submersion in hot water at 55.degree.-65.degree. C. for 1-4 hours, and such conditions are representative of institutional cooking requirements. Submersion at 70.degree.-100.degree. C. for up to about 12 hours probably represents the limiting case. Under such conditions, a cook-in packaging material should maintain seal integrity, i.e. any heat sealed seams should resist being pulled apart during cook-in. As a corollary, the film is heat sealable to itself. Additionally, the packaging film is substantially conformable to the contained food product. Preferably, this substantial conformability is achieved by the film being heat shrinkable under these conditions so as to form a tightly fitting package. In other words, in an advantageous embodiment, the film is heat shrinkable under these time-temperature conditions, i.e. the film possesses sufficient shrink energy such that submerging the packaged food product in hot water will shrink the packaging film snugly around the contained product, representatively up to about 55% monoaxial or biaxial shrinkage.
Also, the film should have food product adherence to restrict "cook-out" or collection of juices between the surface of the contained food product and the food contact surface of the packaging material during cook-in, thereby increasing product yield. More particularly, in the types of multi-layer films wherein the first "sealing and food contact" layer is of the type of material that bindingly adheres to a contained food product during cook-in, this first layer may alternatively be referred to as the "adhering layer". As used herein, the term "adhere" is intended to mean that the food contact surface of the film bonds during cook-in to the contained food product to an extent sufficient to substantially prevent accumulation of fluids between the film and the contained product.
A heat-shrinkable, cook-in film is described in U.S. Pat. No. 4,469,742 (1984) to Oberle et al. This patent relates to a cook-in shrink film that includes a first "sealing or food contact" layer of nonlipophillic polymeric material having a softening point greater than that of the following shrink layer; a second or shrink controlling layer, melt bonded to the first layer, of an ethylene homopolymer or copolymer; a third or adhesive layer, melt bonded to the second layer, of a chemically modified polyethylene being irradiatively cross-linkable and having functional groups with a relatively strong affinity for the following barrier layer; a fourth or barrier layer, melt bonded to the third layer, of a hydrolyzed ethylene-vinyl acetate copolymer; a fifth or adhesive layer as in said third layer, melt bonded to the fourth layer; and a sixth or abuse layer, melt bonded to the fifth layer. In one embodiment, the first "sealing and food contact" layer is a metal salt neutralized copolymer of an olefin and a carboxylic acid, representatively Surlyn (.TM.). This patent explains Surlyn is the type of material that bindingly adheres to a contained meat product during cook-in. Thus, this Surlyn layer also functions as a protein-adhering layer. The patent also describes a method for making the film including full coextrusion and selective irradiation and orientation.
In the conventional method of manufacturing heat shrinkable film as described in the Oberle et al patent, a tubular orientation process is utilized wherein a primary tube of the film is biaxially oriented by stretching with internal pressure in the transverse direction and with the use of pinch rolls at different speeds in the machine direction. Then the stretched bubble is collapsed, and the film is wound up as flattened, seamless, tubular film to use later to make bags, e.g. either end-seal bags typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bag bottom, or side-seal bags in which the transverse heat seals form the bag sides and one edge of the tubing forms the bag bottom. Such bags are typically used by placing the food product in the bag, evacuating the bag, either heat-sealing the bag mouth or gathering and applying a metal clip around the gathered mouth of the bag to form a seal, and then immersing the bag in hot water at approximately the same temperature at which the film was stretch-oriented, typically about 160.degree. to 205.degree. F. (61.degree. to 96.degree. C.), hot water immersion being one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly. Alternatively, the bag may serve as a liner of a cooking mold.
Also of interest is a plastic, adhering cook-in package such as the casing described in U.S. Pat. No. 4,606,922 (1986) to Schirmer, relating to a method for enhancing yield of a cook-in packaged meat product that includes first providing an adhering cook-in container including a flexible thermoplastic envelope being substantially conformable to a contained meat product and having an inner meat-contacting surface of a selectively irradiated ionomer of a metal salt neutralized copolymer of ethylene and acrylic acid or meth-acrylic acid, then conforming the container about a selected meat product and cooking the packaged product, whereupon the inner surface of the envelope bonds to the meat product substantially to prevent cook-out of fluids. Representatively, the ionomer of the inner binding surface is Surlyn, and a typical casing or envelope is of the structure: nylon 6/adhesive/Surlyn.
Also of interest is a flexible plastic, adhering cook-in package described in U.S. Pat. No. 4,411,919 (1983) to Thompson, relating to a method for enhancing yield of cook-in packaged meat product, comprising: (a) providing an adhering cook-in package comprising a flexible plastic container being substantially conformable to a selected meat product and having an inner meat product contacting surface of polymeric olefin having been subjected to an energetic radiation surface treatment in the presence of oxygen sufficient to cause said inner surface to adhere to the meat product during cook-in, said container having been formed from hot blown tubular film; (b) conforming said package about a selected meat product; and (c) cooking the packaged product, whereupon said inner surface adheres to said meat product to substantially prevent cook-out of fluids therefrom.
Also of interest is the film described in U.S. Pat. No. 4,303,711 (1981) to Erk and Korlatzki, which relates to a tubular film consisting of biaxially stretched plastic material for packing and casing paste-type foodstuffs that either, after packing, are heated or are packed in a hot fluid state, comprising a mixture of approximately 50-99 parts by weight of at least one aliphatic polyamide having a glass transition point in the dry state of at least 48.degree. C., and a glass transition point after moisture absorption of 3.degree. C. or less and approximately 1-50 parts by weight of one or more members of the group consisting of an ionomer resin, a modified ethylene/vinyl acetate acid copolymer and a modified polyolefin. Another patent to Erk and Korlatzki is U.S. Pat. No. 4,601,929 (1986), relating to a single layer of polyamide film for packing and casing foodstuffs in paste form, especially foodstuffs that are packed when hot or are subject to heat treatment after packing.
Also of interest is the film described in U.S. Pat. No. 4,568,580 (1986) to Ghiradello et al, relating to an article of manufacture for packaging food products comprising: (a) a first film section, said section having at least one surface comprising a copolyamide obtained by random copolymerization of precursor monomers of at least two different polyamides, said copolyamide having a melting point measured on a Perkin Elmer DSC-2 device in the range from 120.degree. C. to 240.degree. C.; (b) a second film section, said section being a section of a film having surface comprising a copolyamide as defined in subparagraph (a) above; and, (c) at least one heat weld between the copolyamide surfaces of said first and second sections thereby forming said articles for packaging food products, said article being capable of withstanding, without suffering damage to said heat weld, heat treatment at temperatures from 70.degree. C. to 120.degree. C. for at least 10 minutes.
Of general interest are the disclosures of U.S. Pat. No. 3,355,319 issued Nov. 28, 1967 to Rees for "Self-Supporting Film With A Heat-Sealable Coating Of An Ionic Copolymer Of An Olefin And Carboxylic Acid With Metal Ions Distributed Throughout" and U.S. Pat. No. 3,845,163 issued Oct. 29, 1974 to Murch for "Blends of Polyamides and Ionic Copolymer". Both of these patents are assigned to E. I. du Pont de Nemours and Company, and relate to metal salt neutralized copolymers of an alpha-olefin having the formula RHC.dbd.CH.sub.2 wherein R is H or C.sub.1 to C.sub.8 alkyl and an alpha,beta-ethylenically unsaturated carboxylic acid. Such materials are marketed by du Pont under the name, Surlyn (.TM.).
The present invention is directed to multiple-layer cook-in film from which packages, such as casings or bags, can be made which exhibit improved heat seal strength, yet still retain at least some food adherence characteristics and often exhibit similar or improved food adherence characteristics as compared to known films having a Surlyn food contact surface that adheres to a food product during cook-in, such as some of the films described in U.S. Pat. No. 4,606,922 and some of the films described in U.S. Pat. No. 4,469,742, both of which are discussed above. The multilayer film structure of the present invention has a "sealing and food contact" layer, more preferably has the minimal structure: (sealing and food contact layer)/(barrier layer), in one advantageous embodiment has the minimal structure: (sealing and food contact layer)/(barrier layer)/(abuse layer), and in another advantageous embodiment has the minimal structure: (sealing and food contact layer)/(shrink layer)/( barrier layer)/(abuse layer), such composite structures being advantageous to achieve the desired composite properties of the packaging film.